The present invention relates to a field emission electron gun and, more particularly, to a field emission electron gun with an evaporation source for an electron microscope.
An electron microscope comprises a field emission electron gun which includes a field emission tip as an electron source, an anode spaced from and facing the field emission tip, and a vacuum chamber. A voltage applied between the tip and the anode draws out electrons from the tip and accelerates the electrons. The applied voltage is on the order of 3 to 5KV. Generally, the anode is at ground potential and a negative high voltage is applied to the field emission tip. The field emission tip is disposed within the vacuum chamber at an ultra high vacuum on the order of 1 .times. 10.sup..sup.-9 Torr in order that it will operate with stability and for a long period of time. The field emission tip is an etched tip which does not employ any filament to emit electrons, the diameter of which is very small, such as 2000 A or smaller. The field emission tip is made of a material such as a tungsten, etc., from which electrons are easily produced.
This type of field emission electron gun has an advantage in that an electron beam of a high electron density is obtained therefrom. Instability, however, arises from excessive gas molecules in the vacuum chamber and the anode surface, to which the gas molecules adhere and are embedded when the electrons from the field emission tip bombard the surfaces thereof during operation of the electron microscope. As a result, the operation of the electron microscope cannot be stably maintained for a long period of time. The larger the beam current from the field emission electron gun, the more conspicuous is this phenomenon. In a scanning type electron microscope, for example, wherein all beam currents emitted from the field emission tip are large (10 to 20 .mu.A), the period of stability is very short (several minutes). Although there are techniques, such as baking, to reduce the gases drawn out of the surfaces of the vacuum chamber and the anode by electron bombardment, a long period of time is required to evacuate the gases from the surfaces thereof and, as a result, such techniques are not practical.